The invention relates to the field of regulation diodes, and more particularly to the diodes ensuring a regulation and a protection function.
A regulation diode is designed to operate in a low power range, for example about one watt, with currents of a few milliamperes and determined breakdown voltages usually ranging from 2 to 200 volts. Such a reverse biased diode operates in the avalanche mode (zener or avalanche effect according to the voltage) with a dynamic resistance as low as possible, that is, the current-voltage characteristic of this diode has to present a marked knee and the voltage has then to remain substantially constant when the current increases.
An exemplary conventional regulation diode is shown in FIG. 1. This diode is formed on a substrate comprising an N-type highly doped layer 1 (N.sup.++) on which is formed a still highly doped N-type layer 2 (N.sup.+), with a doping level chosen as a function of the desired regulation voltage. From the upper surface of this substrate is formed a P-type layer 3. The window surface from which is formed layer 3 is reduced and calculated as a function of the current density to be obtained during the regulation mode. Usually, the junction between layer 3 and layer 2 is surrounded by a guard ring 4 made of a region of the same conductivity type as layer 3 but having a lower doping at the interface between this layer and the N.sup.+ region. This guard ring is designed to ensure the proper operation of the desired zener or avalanche phenomenon while avoiding the problems inherent to the junction curvature at the periphery. Then, the upper surface is coated with a metallization 5 as well as the lower surface (not shown).
In such a zener diode, the avalanche or regulation voltage is mainly determined by the doping level of the N.sup.+ -type layer 2 and only at the second order by the gradient of the PN junction and the specific shape of this junction (planar or rounded up junction). Thus, when it is desirable to obtain regulation diodes corresponding to different voltages, doping levels 2 of various levels are chosen. For example, for a 3-volt regulation voltage, the resistivity of the N.sup.+ -type layer 2 will be about 6 milliohms.cm (about 10.sup.19 atoms/cm.sup.3); for a 51-volt regulation voltage, this resistivity will be 300 milliohms.cm; and for a 200-volt regulation voltage, this value will be 2.5 ohms.cm (about 2.times.10.sup.15 atoms/cm.sup.3).
On the other hand, a zener diode structure such as the one illustrated in FIG. 1 is fragile when current pulses having a value of several amperes are applied because the current density in the active area (interface between layers 3 and 2) becomes too high and heating becomes important.
Clipping protection diodes are also manufactured, the purpose of which is to withstand very high instantaneous overcurrents or overvoltages while bearing reverse overcurrent of several amperes. For that purpose, it is desirable to enhance the thermal dispersion by distributing the heating on the largest possible surface. One will then obtain, unlike regulation diodes, large surface junctions.
It has been proposed in the prior art to combine the regulation and protection functions by associating a low surface regulation diode with a larger surface protection diode operating at a threshold slightly higher than that of the regulation diode, the latter diode relaying the former one when an overvoltage occurs.
An exemplary conventional diode is illustrated in FIG. 2. In this figure, the same reference numerals designate the same elements as in FIG. 1. In addition to the junctions already represented in FIG. 1, there is provided a P-type area 6 having a relatively large surface surrounding the area of the regulation diode 3 and positioned between this diode and the guard ring 4 (in some implementations, no guard ring is provided). J1 designates the regulation junction between the diffused area 3 and the N.sup.+ -type layer 2, and J2 designates the protection junction between the diffused area 6 and this layer 2.
The manufacturing of such a structure involves numerous technological problems since it is mandatory to set in a reproducible way the avalanche voltages V1 and V2 associated with the two junctions J1 and J2. In fact, it is necessary to carefully set voltage V1 corresponding to junction J1 for determing the desired regulation voltage. Then, in order to obtain a satisfactory protection performance, it is necessary to set in a predetermined way the difference in voltage V2-V1 which defines the protection performance.
Conventionally, a structure such as the one of FIG. 2 is made from an N.sup.+ -type substrate corresponding to the chosen doping level for layer 2 and the rear surface is more highly doped for forming the N.sup.++ -type layer 1 designed to reduce the diode resistivity and to improve the ohmic contact on its rear surface. Then, the junctions J1 and J2 are formed for example by diffusing in a first step the protection ring 6 from a solid doping source. The protection ring voltage is determined at the first order by the annealing time duration. A diffusion of the central junction 3 is then carried out also from a solid source.
The realization of the guard ring, which is carried out in a conventional way, has not been described hereinabove.
Of course, according to the technologies, diffusion processes other than the one resulting from a solid dopant source diffusion are also liable to be used, for example gaseous diffusions, or implantations followed by annealing processes.
Those technologies present various drawbacks and limitations, among which:
1) Since the doping level of the N-type layer 2 determines at the first order the regulation voltage, the manufacturer who desires to supply a full range of diodes corresponding to different regulation voltages will have to keep on the shelf silicon in a large resistivity range, each resistivity corresponding to a desired regulation voltage, which involves important stocking problems.
2) On a given silicon wafer, there is a resistivity dispersion that is liable to amount to 20% (ingot striation) which may entail a dispersion of the regulation voltages up to 10%. As a result, sorting out by the manufacturer causes a manufacturing output drop.
3) Further to the choice of the regulation voltage, for determining the protection voltage, it is necessary to choose the diffusion time duration of the corresponding P-type layer (layer 6). This is empirically carried out by previously establishing a table of the diffusion time durations, a protection voltage corresponding to each time duration. With such a process it is difficult to choose a properly determined difference between the protection and regulation voltages. In other words, for each resistivity value of layer 2, it is necessary to select the diffusion time durations of the areas corresponding to the regulation junction and protection junction, which is a complex technological operation.